WASHINGTON (Reuters) - Less than a third of U.S. children are taking vitamin and mineral dietary supplements, according to a study published on Monday by researchers who were surprised the number was so low.

All told, 32 percent of U.S. children used a dietary supplement in the past month, based on a nationally representative survey from 1999 to 2002 that included 10,136 children age 18 or younger, the researchers said.

The most commonly used supplements were multivitamins and multiminerals, taken by 18 percent of the children. Another 4 percent used single-vitamin supplements and 2 percent used single-mineral supplements, and just under 1 percent used botanical supplements, the researchers said.

The remaining supplement users took a diverse array of other supplement types, the researchers said.

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"In the adult population, 50 percent of the U.S. population is taking any dietary supplement. Thirty percent of the adult population is taking a multivitamin, multimineral preparation," Mary Frances Picciano, a nutrition researcher at the National Institutes of Health who led the study, said in a telephone interview.

"We anticipated that the usage would be higher among children than it would be among adults, and we found just the opposite," Picciano added.

The study appears in the journal Archives of Pediatrics & Adolescent Medicine.

The children using supplements were more likely to be thinner, from a higher-income family without smokers, and spend less time with television and video games, the study found. Non-Hispanic white children were about twice as likely as black children to take these supplements.

Among age groups, the most likely to be taking a dietary supplement were those ages 4 to 8 (41 percent), followed by ages 1 to 3 (38 percent), ages 9 to 13 (29 percent), ages 14 to 18 (26 percent) and infants younger than a year old (12 percent), the study found.

Picciano said she was particularly surprised at how few infants and young children were taking supplements. She noted, for example, that the American Academy of Pediatrics recommends vitamin D supplements for infants who are breast-fed.

The researchers said leading medical groups emphasize a proper diet as the best source of nutrition for children.

"A dietary supplement cannot make up for a faulty diet. The diet still has to contain all the food groups," Picciano said.

"Yet, there are certain nutrients that cannot be reliably provided by the diet," Picciano added. "And vitamin D and calcium are classic examples."